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A buoyant idea

By Jeremy Webb

The Eureka moment came during a sailing regatta off the Norwegian coast. Dag Christensen and Knut Solberg were sitting on deck, waiting for the wind to pick up, when they saw a buoy bobbing up and down in the water. The friends, who work for the global energy company Statoil, made a sketch on a napkin and took it to their employer. “They asked themselves&colon; ‘What if that buoy were 100 metres tall and had rotating blades on top?'” says Sjur Bratland of Statoil. The company has spent the past decade turning that sketch into the 5,300-tonne, 2.3-megawatt prototype that now sits off the coast of Norway. It is the world’s first full-scale floating wind turbine.

Hywind, as it is known, launched in December 2009. “We’re getting superb results because Hywind is located far out at sea in deep water, where the winds blow more powerfully and reliably than they do near shore or on land,” says Bratland, who manages the project.

Conventional offshore wind turbines tend to be located in water no deeper than 20 metres because of the cost of manufacturing and installing the giant steel columns that secure them to the seafloor. In place of this, Hywind has a ‘floater’&colon; a 100-metre-long hollow steel tube that is weighted at the bottom and floats vertically beneath the sea surface.

Finnish engineering firm Technip manufactured the floater and towed it to a quay near Stavanger in June 2009, where it was upended and weighed down with gravel. Bratland’s team then bolted the 65-metre-high tower on top of it. Lastly, a nacelle and 80-metre-diameter blades were installed. This is in contrast to most offshore turbines, which are assembled at their final destination, where rough conditions slow construction and drive up costs. “We sidestepped that issue by doing most of the hard work in the quay,” says Bratland.

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Once assembled, Hywind was towed out to sea, 10 kilometres southwest of the island of Karmøy, west of Stavanger. “We chose this site because it’s so stormy,” says Bratland. “If Hywind can survive here, it can survive anywhere.” The turbine was tethered by slack cables to three giant concrete blocks that sit on the seafloor. These allow the turbine to drift by eight metres in any direction. Although it is battered by gales and high waves, Hywind never sways by more than three degrees from vertical. It is kept stable by its low centre of gravity and by software that feathers the turbine blades if the wind grows too strong. In the event that wind speed exceeds 25 metres a second, Hywind is programmed to shut down, but this does not happen often, nor has it significantly affected its electricity output.

Indeed, since being installed, the turbine has produced about 40 per cent of the 2.3 megawatts it would generate if the wind blew hard all the time. In March, it reached 46 per cent. “Given that the UK average is 30 per cent and that 40 per cent was our target, we’re really impressed by its performance,” says Bratland.

Hywind is only halfway through a two-year test period, but Statoil has already learnt lessons about how to reduce costs. The floater on the next generation of turbines will contain less steel; it may even be replaced by a cheaper concrete model. Bratland is also planning to replace the huge drag anchors with cheaper ‘torpedoes’&colon; iron weights that, when dropped onto the seabed from a height of 50 metres, burrow deep into the mud.

Double the power

Most importantly, Hywind’s descendants will boast turbines that are capable of generating five megawatts of power, thus reducing costs and increasing income. The new turbines will need to be lighter than Hywind’s existing 200-tonne turbine. Conventional turbines with gear boxes are too heavy, so Statoil is exploring direct-drive models and other turbines driven by hydraulic fluid or permanent magnets. “These are the main contenders, but I’m still reading the papers, waiting for the perfect one,” says Bratland.

Many of these design changes will be piloted when Statoil starts to build the world’s first floating wind farm in two or three years. If this 15-megawatt array is a commercial success, Statoil hopes to build similar farms in Spain, Japan and other countries where the ocean drops away quickly into deep water.

Even countries surrounded by shallow water can benefit from Hywind, says Phil de Villiers, an offshore wind manager at the UK government’s Carbon Trust. If the UK is to meet its aim of getting 15 per cent of energy from renewable sources by 2020, the contribution of offshore wind to the electricity mix must jump to 25 per cent. This will mean installing turbines at a rate of one a day between 2010 and 2016, and 2.5 a day after that. “After 2025, the UK will start to run out of shallow water in which to build these turbines. That’s when Hywind will come to the fore,” says de Villiers.

Hywind isn’t the only floating wind turbine being tested – there are two others – but it is far and away the most sophisticated, says Paul Reynolds of RenewableUK, which represents the wind and marine energy industries. Particularly impressive is its stability, says de Villiers. “Hywind has a well-engineered control system that ensures the turbine remains close to vertical,” he says. “It has exceeded expectations.” Hywind isn’t Statoil’s only investment in wind power. The company is also developing two large conventional offshore wind farms in the North Sea at Sheringham Shoals and Dogger Bank.

Although aware of its environmental responsibilities, Statoil is not dogmatically ‘green’, Bratland insists. “We are a business, which means we only invest in technologies that are profitable,” he says. “And it’s clear that offshore wind is going to be hugely so.”